Recent advances in IC design and fabrication make possible the integration of digital and analog circuits on the same IC chip. This technology is widely used in mobile communication systems where a digital core is combined with analog RF circuits. The digital and analog circuits are typically formed as a variety of components near one surface of a wafer. These components may be at several levels separated by inter-metal dielectric layers. Usually, the topmost layer is made of a dielectric material and serves as a passivation layer for the entire structure.
The integration of the digital and analog circuits causes noise coupling between the digital and analog circuits. The analog circuit is especially affected by the noise generated in the digital circuit. This significantly limits the performance achieved in analog signal processing and data conversion circuits, such as differential amplifiers that are extremely sensitive to the noise at the differential inputs. FIG. 1 illustrates the noise path between the digital and analog circuits. Region 4 is a digital circuit. Region 6 is an analog circuit. Arrows 8, 10 and 12 symbolize one of the noise paths in the substrate 2.
Besides the noise interference between the digital circuit and analog circuit, noise interference also exists between the digital circuit components.
There is a significant dependence of the noise coupling through the substrate on the constitution of the silicon substrate. Therefore, various methods have been developed to break the noise path in the silicon substrate. One commonly used method is forming isolation layers in the substrate. As shown in FIG. 1, an isolation layer 14 breaks the noise path between circuit regions 4 and 6. Isolation layer 14 is typically formed of dielectric materials. One example of the isolation layer 14 is a trench isolation between the circuits to be isolated. To form deep trench isolation, trenches with near vertical sides are etched between the circuits and then filled with dielectric materials.
However, even deep trench isolation is not fully satisfactory when full isolation between the circuits is required. This is particularly true when high-speed analog circuits are involved.
Another known method is placing a guard ring in the substrate and between the circuits to be isolated. As illustrated in FIG. 2, a p+ guard ring 20 is formed in a p− substrate 2. The guard ring 20 is grounded as shown at 22; therefore, it creates a low resistivity path for the substrate noise. The noise is more likely to take the low resistivity path to the guard ring 20 than a higher resistivity path to another circuit region.
Yet another method has been developed. FIG. 3 illustrates a proton bombardment approach. Semi-insulating region 24 is created by proton bombardment from the topside of the substrate 2 between the circuits 4 and 6 to be isolated. The semi-insulating region 24 has resistivity of higher than about 105 Ω-cm. Therefore, a high resistivity path is created between the circuits to isolate the noise. To further isolate the circuits, the substrate 2 is also bombarded from the backside, creating a semi-insulating region 26. When the semi-insulating region 24 created from the topside and the semi-insulating region 26 created from the backside of the circuit are connected, the noise path in the substrate is effectively isolated by high resistivity isolations.
The guard ring and proton bombardment are effective methods for noise isolation. However, when the size of the integrated circuit drops to 0.13 μm or lower, and the frequency increases to over about 1 GHz, the noise interference becomes more severe and better isolation techniques are needed.